A process for producing nitrous oxide comprises reacting ammonia with nitric oxide and/or oxygen in the presence of a catalyst comprising a Group VIB metal oxide, to produce a reaction mixture comprising nitrous oxide, and optionally recovering the nitrous oxide from the effluent mixture....http://www.google.com/patents/US6312657?utm_source=gb-gplus-sharePatent US6312657 - Production of nitrous oxide

A process for producing nitrous oxide comprises reacting ammonia with nitric oxide and/or oxygen in the presence of a catalyst comprising a Group VIB metal oxide, to produce a reaction mixture comprising nitrous oxide, and optionally recovering the nitrous oxide from the effluent mixture.

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Claims(5)

What is claimed is:

1. A process for producing nitrous oxide comprising the step of reacting ammonia with nitric oxide and/or oxygen in the presence of a catalyst comprising a Group VIB metal oxide on a support comprising zirconia, to produce an effluent mixture comprising nitrous oxide, wherein the catalyst is produced by precipitation of said Group VIB metal oxide from solution onto said support to form a catalyst precursor and then calcining the catalyst precursor at a temperature of about 500° C. to about 850° C.

2. The process of claim 1 and including the further step of recovering the nitrous oxide from the effluent mixture.

3. The process of claim 1 wherein the Group VIB metal oxide of the catalyst is molybdenum oxide.

4. The process of claim 1 wherein said step of reacting ammonia with nitric oxide and/or oxygen in the presence of a catalyst is carried out at a temperature of 400° to 500° C.

5. The process of claim 1 wherein said catalyst further comprises iron oxide.

Description

FIELD OF THE INVENTION

This invention is directed to the production of nitrous oxide, N2O, from ammonia and nitric oxide, NO, over a catalyst comprising a Group VIB metal oxide. Specifically, the catalyst of this invention comprises an oxide of a Group VIB metal such as molybdenum oxide, that may optionally be supported on a Group VIB metal oxide support such as zirconia, or on a solid support material such as silica, and that may optionally comprise a Group VIIIB metal oxide component such as iron oxide.

BACKGROUND OF THE INVENTION

One commercial process for making nitrous oxide involves the thermal decomposition of ammonium nitrate when heated from 200 to 260° C. according to the following equation:

NH4NO3—N2O+2H2O

However, extreme caution must be used in handling ammonium nitrate, which may be highly explosive under extreme shock or elevated temperatures. Therefore, improved methods for controlling the thermal decomposition of ammonium nitrate have been disclosed. For example, the thermal decomposition of ammonium nitrate in the presence of a melt containing ammonium hydrogen sulfate and ammonium sulfate is described in U.S. Pat. No. 4,154,806; the thermal decomposition of ammonium nitrate into an aqueous, strongly acid reaction liquor containing chloride ions as a catalyst is described in U.S. Pat. No. 4,102,986; and the thermal decomposition of ammonium nitrate in a chloride-containing aqueous solution of nitric acid and the presence of catalytically active ions of manganese, copper, cerium, lead, bismuth, cobalt or nickel, is described in U.S. Pat. No. 3,656,899.

Another commercial process for producing nitrous oxide involves the reaction of ammonia and air using Mn and Bi oxides as catalysts. For example, Japanese Patent No. 6122507 describes a process for preparing nitrous oxide by the oxidation of ammonia with oxygen in the presence of steam and a CuO/MnO2 catalyst. Other methods for producing nitrous oxide include the reaction of a molten nitrate salt with ammonium chloride and the reaction of ammonia with at least one molten nitrate salt of an alkaline earth metal, as described in U.S. Pat. No. 4,720,377.

The present invention is directed to the conversion of ammonia with nitric oxide and/or oxygen into nitrous oxide using a catalyst comprising a Group VIB metal oxide. Accordingly, the present invention avoids the dangers associated with the use and handling of ammonium nitrate and is an alternative and novel nitrous oxide synthesis method that is inexpensive and safe.

SUMMARY OF THE INVENTION

The present invention is directed to a process for producing nitrous oxide comprising the step of reacting ammonia with nitric oxide and/or oxygen in the presence of a catalyst comprising a Group VIB metal oxide, to produce an effluent mixture comprising nitrous oxide.

DETAILED DESCRIPTION OF THE INVENTION

The Catalyst

One embodiment of the present invention utilizes the Group VIB metal oxide alone as the catalyst for reaction.

A second embodiment utilizes a catalyst that comprises the Group VIB metal oxide in combination with the Group VIIIB metal oxide.

A third embodiment utilizes a catalyst that comprises the Group VIB metal oxide, in combination with the Group IVB metal oxide as a support.

A fourth embodiment utilizes a catalyst that comprises the Group VIB and Group VIIIB metal oxides, in combination with the Group IVB metal oxide as a support.

A fifth embodiment utilizes a catalyst that comprises the Group VIB metal oxide, in combination with a solid support material.

A sixth embodiment utilizes a catalyst that comprises the Group VIB and Group VIIIB metal oxides, in combination with the solid support material.

The Group VIB metal oxide of the present invention may be an oxide of chromium, molybdenum or tungsten. Preferably, the Group VIB metal oxide is molybdenum oxide, MoO3. Sources of the Group VIB metal oxide may include pure molybdenum oxide MoO3, ammonium heptamolybdate tetrahydrate (NH4)6Mo7O24. 4H2O, and molybdenum halides and oxyhalides.

The Group IVB metal oxide of the present invention may be an oxide of titanium, zirconium or hafnium. Preferably, the Group IVB metal oxide is zirconia. Sources of the Group IVB metal oxide may include zirconyl chloride and zirconyl nitrate. When the Group IVB metal oxide is used in combination with the Group VIB metal oxide, the weight ratio of the Group VIB metal oxide to Group IVB metal oxide is from about 0.001:1 to 0.5:1, and preferably from about 0.1:1 to 0.3:1.

Examples of the solid support materials include silica and the material designated M41S, which is described in U.S. Pat. No. 5,102,643. A particular example of such an M41S material is a material designated MCM-41, which is described in U.S. Pat. No. 5,098,684. When the Group VIB metal oxide is used in combination with the solid support material, the weight ratio of the Group VIB metal oxide to the solid support material is from about 0.05:1 to 0.8:1, and preferably from about 0.15:1 to 0.6:1.

The Group VIIIB metal oxide may be an oxide of iron. Preferably, the Group VIIIB metal oxide is iron oxide. Sources of the Group VIIIB metal oxide may include iron (II) sulfate heptahydrate FeSO4.7H2O, iron halides and iron nitrate. When the Group VIIIB metal oxide is used in combination with the Group VIB metal oxide alone, the Group VIB and Group IVB metal oxides, or the Group VIB metal oxide and the solid support material, the molar ratio of the Group VIIIB metal oxide to the Group VIB metal oxide is from about 0.001:1 to 0.1:1, and preferably from about 0.005:1 to 0.05:1.

Preparation of the Catalyst

The catalyst of the present invention may be prepared, for example, by combining a first liquid solution comprising a source of the Group VIB metal oxide, and a second liquid solution comprising a source of the Group IVB metal and/or a third liquid solution comprising a source of the Group VIIIB metal oxide, under conditions sufficient to cause precipitation of the catalyst Examples of the precipitating reagent include ammonium hydroxide, alkylammonium hydroxide and alkaline hydroxides such as sodium hydroxide. Water is a preferred solvent for these solutions.

Optionally, the liquid solution from which the catalyst is precipitated may comprise a solid support material, in which case the components of the catalyst may be precipitated directly onto the solid support material. For example, the Group VIIIB metal oxide may optionally be co-precipitated from the liquid solution, along with the Group VIB metal oxide, directly onto the solid support material.

The temperature at which the liquid solution is maintained during precipitation may be less than about 200° C., e.g., from about 0° to 200° C. A particular range of such temperatures is from about 50° to 100° C. The pH range of the liquid solution during precipitation is from about 4 to 11.

The catalyst may be recovered from the liquid solution by filtration, followed by drying. The catalyst may be subjected to a final calcination as described below to dehydrate the catalyst and to confer the required mechanical strength on the catalyst.

Calcination of the catalyst may be carried out, preferably in an oxidizing atmosphere, at about atmospheric pressure to 6890 kPa (about 1000 psi); and at temperatures from about 500° to 850° C. The calcination time may be up to 48 hours, e.g., for about 0.5 to 24 hours, e.g., for about 1.0 to 10 hours.

The Process

Nitric oxide and ammonia are converted to nitrous oxide in the present invention by a reaction which may be described by the following equation:

4NH3+4NO+3O2—4N2O+6H2O,

although other reactions may also occur.

An additional reaction that occurs when oxygen is present is the oxidation of ammonia with oxygen to form nitrous oxide which may be described by the following equation:

2NH3+2O2—N2O+3H2O

The molar feed ratio of nitric oxide to ammonia ranges from about 0.01:1 to 10:1; the amount of oxygen fed to the reaction unit ranges from about 0.01 to 35 molar %.

The method according to the present invention is carried out at a temperature ranging from about 200 to 600° C., and preferably from about 300 to 500° C.; a pressure ranging from about 0.1 to 100 atmospheres, and preferably from about 0.5 to 5 atmospheres. The gas hourly space velocity for the reaction ranges from about 1,000 to 10,000,000 hr−1, and preferably from about 20,000 to 1,000,000 hr−1.

The effluent mixture comprises nitrogen, oxygen, nitric oxide, ammonia and the desired product, nitrous oxide. The nitrous oxide may be separated and recovered from the effluent mixture by conventional means known to skilled artisans or the entire effluent may be used as a feed to a separate reactor. Alternatively, the nitrous oxide can be utilized in situ as a selective oxidant for various substrates. For example, benzene could be co-fed with the reactants and phenol product recovered.

The invention will now be more particularly described with reference to the Examples, each of which discloses the conversion of a gaseous mixture containing ammonia together with nitric oxide and/or oxygen, with the balance in each case being helium.

EXAMPLE I

Five hundred grams of ZrOCl2.8H2O were dissolved with stirring in 6.0 liters of distilled water. To this, a solution containing 33 grams of (NH4)6Mo7O24.4H2O in 500 ml of H2O was added. Finally, a solution containing 263 ml of conc. NH4OH and 500 ml of distilled H2O was added dropwise over a 30-45 minute period. The pH of the solution was approximately 9. This slurry was then placed in a steambox for 72 hours. The product formed was recovered by filtration, washed with excess water, and dried overnight at 85° C. The material was then calcined in air at 700° C. for 3 hours. Chemical analysis of the dried materials showed an Mo/Zr ratio of 0.11 (weight basis); this corresponds to a water-free sample composition of approximately 7.3% Mo.

Table 1 shows the activity and selectivity of this sample for the conversion of 500 ppm each of ammonia and nitric oxide into N2O at a space velocity of approximately 600,000 hr−1. At 500° C., about 60% of the inlet ammonia and nitric oxide is converted. The selectivity to N2O is approximately 61%; the remaining 39% of the material is converted to N2.

TABLE 1

Conversion of NO and NH3 to N2O over catalyst described in Example I.

0.050 g catalyst used. Space velocity˜600,000 hr−1.

inlet concentrations

outlet concentrations

selectivity to

T(° C.)

O2(%)

NH3(ppm)

NO(ppm)

NH3(ppm)

NO(ppm)

N2(ppm)

N2O(ppm)

N2O

300

1

500

500

448

447

45

5

9.9%

350

1

500

500

399

397

84

14

14.4%

400

1

500

500

345

345

105

44

29.6%

450

1

500

500

273

288

109

107

49.7%

500

1

500

500

173

230

115

180

61.1%

EXAMPLE II

This catalyst was prepared in a manner identical to Example I except that it was calcined in air at 830° C. for 3 hours. Table 2 shows that calcination of this material at a higher temperature gives higher selectivity to N2O at temperatures of 400° C. and below than the catalyst of Example I, but a lower selectivity at 500° C. The selectivity of this sample is much less affected by temperature than that of the catalyst used in Example I.

TABLE 2

Conversion of NO and NH3 to N2O over catalyst described in Example II.

0.050 g catalyst used. Space velocity˜600,000 hr−1.

inlet concentrations

outlet concentrations

selectivity to

T(° C.)

O2(%)

NH3(ppm)

NO(ppm)

NH3(ppm)

NO(ppm)

N2(ppm)

N2O(ppm)

N2O

300

1

500

500

465

473

14

16

52.7%

350

1

500

500

397

432

37

47

55.7%

400

1

500

500

291

397

70

85

54.8%

450

1

500

500

193

378

101

110

52.2%

500

1

500

500

132

378

124

117

48.5%

EXAMPLE III

Five hundred grams of ZrOCl2.8H2O were dissolved with stirring in 6.0 liters of distilled water. To this, a solution containing 66 grams of (NH4)6Mo7O24.4H2O in 500 ml of H2O was added. Finally, a solution containing 263 ml of conc. NH4OH and 500 ml of distilled H2O was added dropwise over a 30-45 minute period. The pH of the solution was adjusted to approximately 9 by the addition of 146 grams of concentrated NH4OH. This slurry was then placed in a steambox for 72 hours. The product formed was recovered by filtration, washed with excess water, and dried overnight at 85° C. The material was then calcined in air at 600° C. for 3 hours. Chemical analysis of the dried materials showed an Mo/Zr ratio of 0.19 (weight basis); this corresponds to a water-free sample composition of approximately 11.4% Mo. Table 3 shows the activity and selectivity of this sample for N2O formation. The higher concentrations of molybdenum gives both higher activities and selectivities than observed in Example I.

TABLE 3

Conversion of NO and NH3 to N2O over catalyst described in Example III.

0.050 g catalyst used. Space velocity˜600,000 hr−1.

inlet concentrations

outlet concentrations

selectivity to

T(° C.)

O2(%)

NH3(ppm)

NO(ppm)

NH3(ppm)

NO(ppm)

N2(ppm)

N2O(ppm)

N2O

300

1

500

500

385

385

91

15

14.5%

350

1

500

500

322

321

121

50

29.0%

400

1

500

500

252

269

109

125

53.5%

450

1

500

500

167

228

95

205

68.4%

500

1

500

500

92

211

99

250

71.7%

EXAMPLE IV

Five hundred grams of ZrOCl2.8H2O were dissolved with stirring in 6.0 liters of distilled water. To this, a solution containing 66 grams of (NH4)6Mo7O24.4H2O in 500 ml of H2O was added. Another solution containing 7.5 grams of FeSO4.7H2O in 500 ml of H2O was added. Finally, a solution containing 263 ml of conc. NH4OH and 500 ml of distilled H2O was added dropwise over a 30-45 minute period. The pH of the solution was adjusted to approximately 9. This slurry was then placed in a steambox for 72 hours. The product formed was recovered by filtration, washed with excess water, and dried overnight at 85° C. The material was then calcined in air at 600° C. for 3 hours. Chemical analysis of the dried materials showed an Mo/Zr ratio of 0.20 (weight basis) and an Fe/Zr ratio of 0.012; this corresponds to a water-free sample composition of approximately 12.1% Mo and 0.72% Fe. Table 4 shows the activity and selectivity of this iron-containing sample.

TABLE 4

Conversion of NO and NH3 to N2O over catalyst described in Example IV.

0.052 g catalyst used. Space velocity˜600,000 hr−1.

inlet concentrations

outlet concentrations

selectivity to

T(° C.)

O2(%)

NH3(ppm)

NO(ppm)

NH3(ppm)

NO(ppm)

N2(ppm)

N2O(ppm)

N2O

300

1

500

500

387

382

90

19

17.2%

350

1

500

500

310

314

117

62

34.4%

400

1

500

500

220

259

110

145

56.9%

450

1

500

500

132

226

101

216

68.1%

500

1

500

500

70

222

106

246

69.8%

EXAMPLE V

250 grams of ZrOCl2.8H2O were dissolved with stirring in 1.5 liters of distilled water. This solution was heated to approximately 60° C. To this, a solution containing 33 grams of (NH4)6Mo7O24.4H2O and 130 grams of conc. NH4OH in 1250 ml of H2O was added. This solution was also heated to approximately 60° C. The solution containing the zirconyl chloride was slowly added to the second solution with mixing. The pH of the solution was approximately 7. This slurry was then placed in a steambox for 72 hours. The product formed was recovered by filtration, washed with excess water, and dried overnight at 85° C. The material was then calcined in air at 600° C. for 3 hours. Chemical analysis of the dried materials showed an Mo/Zr ratio of 0.24 (weight basis); this corresponds to a water-free sample composition of approximately 14.1% Mo. Table 5 shows that catalyst synthesis at a higher temperature results in a sample with slightly higher selectivity to N2O, especially at temperatures of 400° C. and below, as compared to Example III.

TABLE 5

Conversion of NO and NH3 to N2O over catalyst described in Example V.

0.064 g catalyst used. Space velocity˜450,000 hr−1.

inlet concentrations

outlet concentrations

selectivity to

T(° C.)

O2(%)

NH3(ppm)

NO(ppm)

NH3(ppm)

NO(ppm)

N2(ppm)

N2O(ppm)

N2O

300

1

500

500

371

362

90

30

25.3%

350

1

500

500

272

279

117

93

44.2%

400

1

500

500

162

223

112

185

62.2%

450

1

500

500

78

202

106

246

69.9%

500

1

500

500

32

208

108

265

71.1%

EXAMPLE VI

This catalyst was prepared in a manner identical to Example V except that it was calcined in air at 700° C. for 3 hours. Table 6 shows that calcining at 700° C. lowers overall activity somewhat compared to Example V; selectivities to N2O are higher at 400° C. and below but are lower at 450 and 500° C.

TABLE 6

Conversion of NO and NH3 to N2O over catalyst described in Example VI.

0.054 g catalyst used. Space velocity˜600,000 hr−1.

inlet concentrations

outlet concentrations

selectivity to

T(° C.)

O2(%)

NH3(ppm)

NO(ppm)

NH3(ppm)

NO(ppm)

N2(ppm)

N2O(ppm)

N2O

300

1

500

500

402

403

62

30

32.8%

350

1

500

500

306

330

89

85

49.0%

400

1

500

500

197

280

98

157

61.7%

450

1

500

500

111

260

106

204

65.9%

500

1

500

500

57

261

115

220

65.8%

EXAMPLE VII

This catalyst was prepared in a manner identical to Example V except that it was calcined in air at 830° C. for 3 hours. Table 7 shows that calcining at 830° C. lowers both activity and selectivity to N2O.

TABLE 7

Conversion of NO and NH3 to N2O over catalyst described in Example VII.

0.052 g catalyst used. Space velocity ˜600,000 hr−1.

inlet concentrations

outlet concentrations

selectivity to

T(° C.)

O2(%)

NH3(ppm)

NO(ppm)

NH3(ppm)

NO(ppm)

N2(ppm)

N2O(ppm)

N2O

300

1

500

500

449

453

29

21

41.7%

350

1

500

500

376

401

54

57

51.1%

400

1

500

500

276

361

77

102

57.0%

450

1

500

500

183

336

97

140

59.2%

500

1

500

500

115

327

112

161

59.0%

EXAMPLE VIII

A solution containing 41.4 grams of (NH4)6Mo7O24.4H2O and 200 grams of H2O was prepared. The solution was slowly added to 150 grams of silica (HiSil 233) and placed in a polypropylene bottle. This mixture was mixed using a roller overnight. The product formed was recovered by filtration, washed with minimum water, and dried overnight at 85° C. The material was then calcined in air at 540° C. for 3 hours. From the preparation we expect this sample to contain approximately 15% Mo by weight. Table 8 shows that this Mo/SiO2 catalyst can also convert nitric oxide and ammonia into N2O, although in this case with a lower selectivity than most of the examples described above.

TABLE 8

Conversion of NO and NH3 to N2O over catalyst described in Example VIII.

0.147 g catalyst used. Space velocity ˜200,000 hr−1.

inlet concentrations

outlet concentrations

selectivity to

T(° C.)

O2(%)

NH3(ppm)

NO(ppm)

NH3(ppm)

NO(ppm)

N2(ppm)

N2O(ppm)

N2O

300

1

500

500

471

472

21

10

31.1%

350

1

500

500

442

442

40

18

31.3%

400

1

500

500

392

401

66

32

32.8%

450

1

500

500

323

351

100

54

35.2%

500

1

500

500

205

280

150

92

38.1%

EXAMPLE IX

Table 9 shows that bulk MoO3, obtained from the Aldrich Chemical Company, is also an effective catalyst for converting nitric oxide and ammonia into N2O. Compared to the supported catalysts, selectivities at 350° C. and below are higher, but fall steadily as temperature increases.

TABLE 9

Conversion of NO and NH3 to N2O over catalyst described in Example IX.

0.266 g catalyst used. Space velocity ˜120,000 hr−1.

inlet concentrations

outlet concentrations

selectivity to

T(° C.)

O2(%)

NH3(ppm)

NO(ppm)

NH3(ppm)

NO(ppm)

N2(ppm)

N2O(ppm)

N2O

300

1

500

500

462

466

10

33

76.0%

350

1

500

500

386

415

29

73

71.4%

400

1

500

500

243

353

72

128

63.9%

450

1

500

500

32

340

151

148

49.5%

500

1

500

500

0

449

194

61

23.8%

EXAMPLE X

The data shown in Table 10 were collected using the catalyst described in Example III. They show that selectivity to N2O is not a strong function of inlet nitric oxide and ammonia concentration when nitric oxide and ammonia are fed in equimolar amounts. There is no reason to believe that feeding much higher concentrations of nitric oxide and ammonia would not also result in the formation of correspondingly high concentrations of N2O.

TABLE 10

Conversion of NO and NH3 to N2O over catalyst described in Example III.

Effect of variations in inlet NO and NH3 concentrations.

0.050 g catalyst used. Space velocity ˜600,000 hr−1.

inlet concentrations

outlet concentrations

selectivity to

T(° C.)

O2(%)

NH3(ppm)

NO(ppm)

NH3(ppm)

NO(ppm)

N2(ppm)

N2O(ppm)

N2O

500

1

500

500

242

311

63

157

71.4%

500

1

200

200

84

132

25

68

73.4%

500

1

350

350

163

220

45

114

71.7%

500

1

750

750

387

463

95

230

70.9%

500

1

907

907

481

560

115

277

71.6%

EXAMPLE XI

The data shown in Table 11 were collected using the catalyst described in Example V. They show that significant amounts of N2O are formed even if nitric oxide and ammonia are not fed in equimolar amounts.

TABLE 11

Conversion of NO and NH3 to N2O over catalyst described in Example V.

Effect of variations in inlet NO and NH3 concentrations.

0.064 catalyst used. Space velocity ˜450,000 hr−1.

inlet concentrations

outlet concentrations

selectivity to

T(° C.)

O2(%)

NH3(ppm)

NO(ppm)

NH3(ppm)

NO(ppm)

N2(ppm)

N2O(ppm)

N2O

500

1

500

500

32

208

108

265

71.1%

500

1

625

375

54

108

148

260

63.7%

EXAMPLE XII

The data shown in Table 12 were collected using the catalyst described in Example IV. They show that some N2O is formed even when no nitric oxide is fed, i.e., when only ammonia and oxygen are fed. Selectivities are lower, however, than when equimolar amounts of nitric oxide and ammonia are fed.

TABLE 12

Conversion of NO and NH3 to N2O over catalyst described in Example IV.

Effect of feeding only NH3.

0.050 g catalyst used. Space velocity ˜600,000 hr−1.

inlet concentrations

outlet concentrations

selectivity to

T(° C.)

O2(%)

NH3(ppm)

NO(ppm)

NH3(ppm)

NO(ppm)

N2(ppm)

N2O(ppm)

N2O

445

1

500

0

211

0

119

17

12.5%

445

2

500

0

212

0

124

20

13.9%

445

3

500

0

214

0

127

22

14.5%

550

1

500

0

48

16

149

61

29.0%

550

3

500

0

51

19

143

78

35.3%

550

0.6

500

0

54

14

146

58

28.5%

550

0.6

1000

0

145

12

316

90

22.2%

550

0.6

1840

0

337

9

606

125

17.1%

EXAMPLE XIII

The data shown in Table 13 were collected using the catalyst described in Example 3 calcined at 600° C. They show that the conversion and N2O selectivity are increased as increasing amounts of oxygen in the 1-4.5% range are fed

TABLE 13

Effect of oxygen concentration on conversion of NO and NH3 over Mo/ZrO2